A composite view showing the rear rate control knob & power switch plus the front LED "strobe" lamp. The knob is directly calibrated in flashes per second The short indicator is the 10's digit.
All the circuitry is built on the small piece of perf board. A small piece of non-conductive bubble wrap (not shown) placed between the removable side and the components keeps everything in place. The case is a small (3-1/4"x2-1/8"x1-1/8") plastic Radio Shack box. It was a #270-something but, the numbers were changed a few years ago.
The Schematic -- A National LM331 precision voltage to frequency converter is the heart of this strobe. The op-amp is a dual rail-rail I/O type. The speed control potentiometer is referenced from the LM331's band bap regulated reference output and then buffered by the first op-amp to provide a 1.9 (at zero Hz) to 0 (at 1KHz) volt speed signal. This sources a current, applied via the calibration R, to the summing node of the second op-amp which is referenced to the LM331's Vref. The LM331 output is current limited and protected so that it may drive the strobe lamp (LED) directly. A series R is included to reduce dissipation in the LM331..